Lessons learned from 43 turbidite giant fields

WORLD TURBIDITES-2

Henry S. Pettingill
Repsol Exploracion SA
Madrid

Part one of this two part article provided a summary of worldwide turbidite exploration and production. This article explores the ingredients that make a giant turbidite field.

There are 43 turbidite fields and discoveries worldwide from which greater than 500 million bbl of oil equivalent are expected to be ultimately recovered (Fig. 1 [134,829 bytes], Table 1 [258,649 bytes]). Some of the giants have not been appraised, so the current reserves figure is subject to change.

However, this limitation will not affect the general observations from the greater data base.

In the 75 years from 1894-1969, approximately 14 billion BOE ultimate recoverable was discovered in 11 turbidite giants. In contrast, in the 28 years since 1970, an additional 34 billion BOE has been added in 30 giants, demonstrating a relatively recent rise in importance of turbidites as hydrocarbon reservoirs (Fig. 2 [50,457 bytes]). Fig. 3 [52,687 bytes] shows this post-1970 surge in giant turbidite discoveries versus the decline in giant field discoveries overall since 1970.

Geology of turbidite giants

This striking increase since 1970 is accompanied by a change in focus of basin type.

A total of 11 giants are from convergent margins, of which 9 are found in "California-type" borderland basins along obliquely convergent margins (Fig. 1 inset). However, the last giant discovered in a borderland basin was in 1936, and the last convergent margin giant was found in 1979.

In contrast, although the first divergent margin turbidite giant was not found until 1949, 30 giants (67% of reserves) have now been discovered in this setting, constituting all but two of the 32 discovered after 1936.

In recent years, Atlantic-type passive margin basins have proven to be the most important setting, producing 15 of the last 18 giant discoveries (Gulf of Mexico, Campos, Niger delta, and Lower Congo basins). In fact, the last eight giants have been found along the Atlantic margin, with the most recent three found in West Africa. The first 10 turbidite giants were discovered onshore, while 30 of the remaining 31 are located offshore.

As is generally true for giant fields in other settings, access to a world-class charge system is a key component. Without exception, the turbidite giants are located in provinces where significant reserves have been proven in other reservoir types, charged from the same source rocks as the turbidites. In fact, many of the large turbidite fields have additional reserves from nonturbidite reservoirs, for example giant Midway-Sunset field in California¹ and the giant Kuito discovery off Angola.²

Reservoir ages of giants range from Permian to Pleistocene (Fig. 4 [37,233 bytes]); however, the greatest opportunity has historically been in the Tertiary, which is productive in 34 of the 43 giants.

Using a simple three-fold classification of trap type for the 35 giants for which trap information is published (Fig. 5 [42,773 bytes]), 25% of the giant reserves come from pure structural traps (nine fields), 10% from stratigraphic traps (four fields), and 65% from combination structural-stratigraphic traps (24 fields), i.e., those which are dependent on both a structural element and a stratigraphic reservoir limit.

A compactional trap component is common among large turbidite discoveries, for example the Frigg complex in the North Sea Viking graben and the Scarborough discovery on the Northwest Shelf of Australia.

In several cases, differential compaction enhances the gross rock volume in closure (and thus field size) by creating an accentuated vertical relief on the top reservoir structure, in some cases when closure does not exist at the base reservoir horizon. Thus, turbidite reservoir geometry is an important control on trapping, even in the case of several structurally-trapped giants.

Field sizes and key volumetric parameters are log-normally distributed with large ranges.³ Two components of gross rock volume, trap area and net pay, are key factors in creating giants.

Trap area ranges from 13 to 350 sq km, and net pay from 20 to 230 m; however, giants with small trap areas always have net pay values exceeding 200 m. Porosities range from 12-35%, permeability from 100 md to 4 darcies, and sustained liquids flow rates from 11,000-18,000 b/d.

The majority have porosities of 25-35%, permeabilities in darcies, and flow rates in excess of 3,000 b/d. Turbidite giants are not unique in this respect, as these parameters are similar to those published for 45 giant fields of North America.⁴ The highest porosity, permeability, and flow rates are generally found in Tertiary reservoirs, whereas the lowest values come from onshore provinces where economics are more favorable.

Development status

As of 1998, 31 of the turbidite giants have been developed. Three giants remain undeveloped 19 years after discovery due to remote location. The largest of this category is the Scarborough gas discovery of the Northwest Shelf of Australia, with 12 tcf recoverable; ⁵ others are located off Venezuela and off northern Canada.

Other less remote discoveries have only recently been developed many years after discovery. For example, in the U.K. North Sea, giant field developments were delayed because of heavy crude and high pressure/high temperature (for example, Captain⁶ and Britannia⁷ fields, respectively).

Several turbidite giants reflect the adage that "a good field just gets better," with ultimate recovery increasing with time. Some of these did not become giants until many years after discovery, when appraisal, satellites, and enhanced recovery provided significant reserve additions. For this reason, several current discoveries less than 500 million BOE have the potential to become giants once fully appraised and once satellite potential is tested.

The future

The recent steep climb in worldwide cumulative reserves indicates that turbidites are in an immature exploration stage globally and will thus play a significant role in the future of hydrocarbon exploration and production.

This analysis of giant fields and the broader analysis of turbidite exploration and production described in Part 1 of this article⁸ indicate that deepwater passive margins will probably provide significant additional turbidite reserves and additional giant discoveries.

A second conclusion is that combination traps and Tertiary reservoirs will be the focus, because they have the potential for large trap areas and/or net pay sections and good quality reservoirs capable of high sustained flow rates. These characteristics have been components of the recent successes in Brazil and West Africa, where several giant discoveries have been made in the last 2 years.

Acknowledgments

The author thanks Repsol Exploracion SA for permission to submit this article for publication. The author is grateful to all the authors who have published papers on the petroleum geology of turbidite fields. Althrough too numerous to list here, they have made an invaluable contribution to the understanding of turbidite exploration and production.

References

1. Lennon, R.B., Midway-Sunset oil field, U.S.A., San Joaquin basin, California, in Foster, N.H., and Beaumont, E.A. (eds.), Atlas of oil and gas fields: Structural traps II: Tectonic fold and fault traps, AAPG Treatise of Petroleum Geology, 1990, pp. 221-241.
2. World Oil, Vol. 218, No. 7, 1997, p. 98.
3. Pettingill, H.S., A historical look at worldwide turbidite production: The importance of stratigraphic traps in predicting play reserves, paper presented at AAPG international convention, Sept. 7-10, 1997.
4. Moody, J.D., Mooney, J.W., and Spivak, J., Giant oil fields of North America, in Halbouty, M.T. (ed.), Geology of giant petroleum fields, AAPG Memoir 14, 1970, pp. 8-18.
5. Kirk, R.B., Submarine fan systems in Australia and New Zealand in a sequence stratigraphic framework-an overview, in Weimer, P., Bouma, A.H., and Perkins, R.F. (eds.), Submarine fans and turbidite systems, sequence stratigraphy, reservoir architecture and production characteristics, Gulf of Mexico and International, GCS-SEPM Foundation 15th annual research conference, 1994, 193-208.
6. Pinnock, S.J., and Clitheroe, A.R.J., The Captain Field, U.K. North Sea: Appraisal and development of a viscous oil accumulation, Petroleum Geoscience, Vol. 3, No. 4, 1997, pp. 305-312.
7. Garret, S., North Sea fields-Britannia, PESGB Newsletter, Aug./Sept. 1997, p. 120.
8. Pettingill, H.S., Worldwide turbidite exploration and production: A globally immature play with opportunities in stratigraphic traps, SPE Paper 49245, SPE annual convention, New Orleans, Sept. 27-30, 1998.
9. Weimer, P., and Link, M.H., Global petroleum occurrences in submarine fans and turbidite systems, in Weimer, P., and Link, M.H., eds., Seismic facies and sedimentary processes of submarine fans and turbidite systems, Springer-Verlag, New York, 1991, pp. 9-67.
10. Johnson, D.S., Rio Vista field-U.S.A., Sacramento basin, Calif., in Foster, N.H., and Beaumont, E.A., eds., Atlas of oil and gas fields, Structural Traps III, AAPG Treatise of Petroleum Geology, 1990, pp. 243-264.

The Author